The invention relates to communications in general. More particularly, the invention relates to a method and apparatus to improve bandwidth of a raman amplifier for use with an optical communications system.
Optical fiber amplifiers are fundamentally important to long-haul optical communications systems. Optical signals begin to attenuate as they travel over an optical fiber transmission medium due to a variety of factors such as fiber loss and dispersion. Optical amplifiers help compensate for such attenuation by providing additional power to the optical signal as it moves through the system.
There are two general classes of optical amplifiers. The first class of optical amplifiers is referred to as lumped amplifiers. Lumped amplifiers linearly increase optical signal power of a supplied input signal via stimulated emission of fiber dopants such as erbium that is subject to an optical pump source. An example of a lumped amplifier would be an Erbium Doped Fiber Amplifier (EDFA). The second class of optical amplifiers is referred to as distributed amplifiers. Distributed amplifiers increase optical signal power along the signal transmission path. An example of a distributed amplifier is a raman amplifier.
Raman amplification is accomplished by introducing the signal and pump energies along the same optical fiber. The pump and signal may be copropagating or counterpropagating with respect to one another. A raman amplifier uses Stimulated Raman Scattering (SRS), which occurs in silica fibers when an intense pump beam propagates through it. SRS is an inelastic scattering process in which an incident pump photon loses its energy to create another photon of reduced energy at a lower frequency. The remaining energy is absorbed by the fiber medium in the form of molecular vibrations (i.e., optical phonons). That is, pump energy of a given wavelength amplifies a signal at a longer wavelength.
FIG. 1 (PRIOR ART) is a plot illustrating the relationship between the pump energy and the raman gain for a silica fiber. The particular wavelength of the pump energy that is used in this example is denoted by reference numeral 1. As shown in FIG. 1, the gain spectrum 2 for this particular pump wavelength is shifted in wavelength with respect to the pump wavelength. Consequently, the bandwidth of the raman amplifier is limited. For example, the bandwidth of the amplifier shown in FIG. 1 is only about 20 nanometers (nm) at a gain of 10 decibels (dB).
FIG. 2 (PRIOR ART) is a plot illustrating the relationship between the pump energy from multiple pumps and the raman gain for a silica fiber. One technique to increase the bandwidth of a raman amplifier is through the use of multiple pumps operating at different wavelengths. As shown in FIG. 2, pump energy supplied at a wavelength denoted by reference numeral 202 generates a gain curve 204 while pump energy supplied at a wavelength denoted by reference numeral 206 generates gain curve 208. The composite gain spectrum, indicated by curve 210, has a bandwidth that is greater than either of the individual gain curves 204 and 208.
Unfortunately, it becomes difficult to increase the bandwidth of a raman amplifier using multiple pumps beyond a limited amount. This limitation arises because it is not possible to provide pump energy that spectrally overlaps the signal. As a result, the wavelength separation between the pump wavelengths 202 and 206 is limited to about 100 nm, since at greater separations the pump wavelength 206 will overlap the gain curve 204 of pump wavelength 202.
In view of the foregoing, it can be appreciated that a substantial need exists for a method and apparatus to increase the bandwidth of a multi-pump raman amplifier without providing pump energy that spectrally overlaps the gain signal.
The embodiments of the invention include a method and apparatus to amplify an optical signal. The optical signal is split into a plurality of parts. Each part is amplified using a separate raman amplifier. Each amplified part is combined into a single amplified optical signal.
With these and other advantages and features of the invention that will become hereinafter apparent, the nature of the invention may be more clearly understood by reference to the following detailed description of the invention, the appended claims and to to the several drawings attached herein.